The present invention relates to processes and apparatus for applying tabs to traveling webs. The invention has particular applicability to the manufacture of disposable diapers.
The history of cutting and applying tape tabs to disposable diaper webs is now entering its fourth decade. Over the course of that time, various types of automatic manufacturing equipment have been developed which produce the desired results with a variety of materials and configurations. This equipment generally included window-knife and slip-and-cut applicators, each having their own advantages and limitations.
Window-knife applicators are comprised of: one or more rotating heads, each made up of a knife edge and a vacuum plate; a more or less stationary knife, which is configured with a hole (window); and a tape transfer mechanism. Typically, the rotating heads are mechanically configured so as to eliminate head rotation relative to the stationary knife. Each head is passed, once per cycle, across the face of the stationary window knife, through which the infeeding tape is passed. The rotating knife shears the extended length of tape against the sharp inner edge of the hole (window), after which the severed segment is held by the vacuum plate. The rotating head, with the segment of tape held in place by the vacuum plate, continues through its rotation to a point, usually 90 degrees later, where it contacts the traveling web, which is pressed against the exposed adhesive of the tape segment. This contact, usually against some backing device, effects a transfer of the tape tab from the vacuum plate to the traveling web, which then carries the tape tab downstream.
Window-knife applicators have a few shortcomings, among which are: the difficulty in feeding tape webs with little axial stiffness; the tendency of the infeeding tape to adhere to the window knife-edge; and for exposed adhesive to contaminate the surfaces of the window knife. For effective cutting, some degree of interference between the cutting edges is necessary between the moving and stationary knife faces, so to minimize impact, precision in manufacturing must be maintained and provision must be made for a degree of resiliency. While applicators of this type have been tested to speeds of 1000 cuts per minute, the maximum practical speed capability of current designs is approximately 750 cuts per minute.
Slip-and-cut applicators are typically comprised of (a) a cylindrical rotating vacuum anvil (b) a rotating knife roll and (c) a transfer device. In typical applications, a tape web is fed at a relatively low speed along the vacuum face of the rotating anvil, which is moving at a relatively higher surface speed and upon which the tape web is allowed to xe2x80x9cslipxe2x80x9d. A knife-edge, mounted on the rotating knife roll, cuts a segment of tape from the tape web against the anvil face. This knife-edge is preferably moving at a surface velocity similar to that of the anvil""s circumference. Once cut, the tape tab is held by vacuum drawn through holes on the anvil""s face as it is carried at the anvil""s speed downstream to the transfer point where the tape segment is transferred to the traveling web.
A common problem with slip-and-cut applicators lies in the tendency to accumulate various contaminants on their anvil surfaces. This is most frequently seen in the form of the release compounds found on the non-adhesive side of tape, which is shipped on pre-wound rolls. Where die-cut tapes are fed onto the surfaces of slip-and-cut applicators, it is common to also see an accumulation of adhesive contamination, as the adhesive has been exposed at the tape edges by the die-cutting process. The difference in speed between the tape web and the anvil tends to xe2x80x9cwipexe2x80x9d adhesive from the tape web. Contamination of the anvil, whether by release compounds or by fugitive adhesive, interferes with the regularity of slip occurring between the tape and the anvil, causing registration and cut accuracy problems. Frequent cleaning is necessary to maintain any level of productivity.
Another problem associated with slip-and-cut applicators occurs at the point of cut. Since the web being cut is traveling at a very low velocity compared to the anvil and knife velocity (perhaps {fraction (1/20)}th), the engagement of the knife with the tape web tends to induce a high tensile strain in the tape web. Having been placed under such a high level of stress, the tape web can recoil violently when the cut is finally completed, causing loss of control of the tape web. This xe2x80x9csnap-backxe2x80x9d effect increases with the thickness of the tape web. Thicker webs tend to prolong the duration of engagement with the knife before completion of the cut, thereby increasing the build-up of strain. This is a common process problem that is usually addressed by the provision of various shock-absorbing devices. One possible solution might have been to reduce the surface velocity of the knife, but substantially different velocities between the knife and anvil result in rapid wear of the knife edge and/or anvil face, depending on relative hardness.
Continual improvements and competitive pressures have incrementally increased the operational speeds of disposable diaper converters. As speeds increased, the mechanical integrity and operational capabilities of the applicators had to be improved accordingly. As a further complication, the complexity of the tape tabs being attached has also increased. Consumer product manufacturers are offering tapes which are die-cut to complex profiles and which may be constructed of materials incompatible with existing applicators. For instance, a proposed tape tab may be a die-profiled elastic textile, instead of a typical straight-cut stiff-paper and plastic type used in the past. Consequently, a manufacturer may find itself with a window-knife applicator, which cannot feed a tape web with too little axial stiffness. It could also find itself with a slip-and-cut applicator, which cannot successfully apply die-cut tape segments. Furthermore, existing applicators cannot successfully apply tapes whose boundaries are fully profiled, as may be desired to eliminate sharp corners, which might irritate a baby""s delicate skin. This demonstrates a clear need for an improved applicator capable of applying new tape configurations and overcoming other shortcomings of prior art applicators.
A basic premise of all applicators using prior art has been to cut the tape at one velocity and then to carry it at its final velocity to the transfer point. The assumption has been made that for correct and accurate placement, the tape tab must be moving at the final web velocity. The proposed invention diverges from that premise, eliminating or reducing the shortcomings associated with prior devices.
In accordance with an important aspect of the invention tape segments are cut and carried at a very low tape web infeed speed. In accordance with a related aspect, problems with transferring a slow-moving segment to a fast-moving web are overcome. Additionally, die-cutting of tape segments to any number of practical shapes is possible, thereby avoiding difficulties associated with prior attempts to do so using previous applicator technology, which required multiple steps to accomplish the same task.
The invention provides the additional benefit of quiet operation compared to prior art equipment, which uses high speed cutting faces and suffers from the effects of the very high energy levels seen at the point of contact. Generally, these energies, and the sounds that they generate, increase in proportion to the square of the velocity. The present invention benefits from the relatively low speed of the cutting faces and exhibits extremely low noise levels. In fact, the underlying noise of the mechanical drive systems and the traveling web equipment contribute to make the cutting noise level nearly unnoticeable.
The present invention provides a simplified process wherein a rotary knife or die, with one or more cutting edges, turns against and in coordination with a corresponding vacuum anvil cylinder. An infeeding tape web is fed along the surface of the anvil, which is rotating at a surface velocity equal to or only somewhat greater than that of the tape web. As the tape web passes the nip created between the knife-edges and the anvil surface, segments of tape are parted but not significantly displaced upon the anvil surface. The segments continue downstream on the anvil surface, held securely by forces induced by a vacuum source directed to one or more holes provided for each segment in the anvil surface.
At a point downstream along the surface of the anvil, the traveling web to which the segments are to be attached is brought into close proximity with the anvil and its tape segments. A mechanically operated device, which may be as simple as a protuberance on a rotating cylinder, presses the target zone of the traveling web against the exposed adhesive of the tape segment as it is presented on the anvil surface. The protuberance preferably has a surface velocity substantially identical to that of the traveling web. Given the extremely low moment of inertia of the tape segment and the aggressive adhesion provided between its exposed adhesive and the compatible surface of the traveling web, each successive segment is successfully transferred to the traveling web, accelerating almost instantly to the speed of the traveling web.
A key aspect of this invention lies in the method and apparatus used to effect the transfer of the tape segments from the anvil to the traveling web. In accordance with the invention, a vacuum commutation system is configured to remove or reduce the level of vacuum used to hold each tape segment to the anvil surface just before the point of transfer. The materials and finishes selected for the anvil and the transfer protuberance provide a situation in which the coefficient of friction between the protuberance and the traveling web is relatively high, while the coefficient of friction between the tape segment and the anvil is relatively low. The highly aggressive nature of the bond between the adhesive side of the tape segment and the target surface of the traveling web ensures that there is virtually no slippage between the two. This ensures that the traveling web is driven through the point of transfer at its existing velocity, and that any tendency of the tape segment to adhere to the anvil surface will not influence the traveling web. The process requires that some slip occurs, and in accordance with the invention, slip occurs only between the tape segment and the anvil surface.
This method is extremely effective in that 25 mm tape segments can be accurately transferred at 800 mm spacing to webs traveling at 300 meters per minute or more. This is a web-to-tape velocity ratio of 32:1. Tape to tape positional accuracy has been found to be extremely precise, with standard deviations of less than 1 mm when applied at a 800 mm spacing. Additionally, a speed capability of more than 2,400 tapes per minute is achievable, easily exceeding the limits of any previously known disposable paper product manufacturing process.